Perpendicularly Magnetized Ferromagnetic Layers Having an Oxide Interface Allowing for Improved Control of Oxidation

ABSTRACT

An improved magnetic tunnel junction with two oxide interfaces on each side of a ferromagnetic layer (FML) leads to higher PMA in the FML. The novel stack structure allows improved control during oxidation of the top oxide layer. This is achieved by the use of a FML with a multiplicity of ferromagnetic sub-layers deposited in alternating sequence with one or more non-magnetic layers. The use of non-magnetic layers each with a thickness of 0.5 to 10 Angstroms and with a high resputtering rate provides a smoother FML top surface, inhibits crystallization of the FML sub-layers, and reacts with oxygen to prevent detrimental oxidation of the adjoining ferromagnetic sub-layers. The FML can function as a free or reference layer in an MTJ. In an alternative embodiment, the non-magnetic material such as Mg, Al, Si, Ca, Sr, Ba, and B is embedded by co-deposition or doped in the FML layer.

RELATED PATENT APPLICATIONS

This application is related to the following: U.S. Pat. No. 8,592,927;and Docket #HT15-006, filing date Nov. 23, 2015, Ser. No. 14/939,232,both assigned to a common assignee and herein incorporated by referencein their entirety.

TECHNICAL FIELD

The invention relates to a composite magnetic structure having acombination (stack) of oxide layers, ferromagnetic layers, andnon-magnetic layers that improve the perpendicular magnetization used inmagnetic thin films such that thermal stability is improved independentof the moment, volume, or crystalline anisotropy for a device withperpendicular magnetic anisotropy.

BACKGROUND

Magnetic thin films magnetized perpendicular to the plane of the filmhave many applications for memory and data storage technologies, e.g.magnetic hard disk drives, Magnetic Random Access Memories (MRAM) ormagnetic domain wall devices.

Perpendicular magnetization relies on Perpendicular Magnetic Anisotropy(PMA), to overcome the magnetostatic shape anisotropy, the favoredin-plane magnetization in thin film geometry.

Several physical phenomena can induce PMA, e.g. crystalline anisotropy,surface or interface anisotropy, and magnetoelastic anisotropy.Interfacial anisotropy occurs at an interface between an Oxide Layer(OL) (e.g. MgO) and a Ferromagnetic Layer (FML) (e.g. Fe, Co, CoFe orCoFeB), and is of particular technological importance. Indeed, thisinterface structure is widely used in MRAM devices, whose memoryelements are based on magnetic tunnel junctions, each having twomagnetic electrodes magnetized perpendicular to the plane of the Siliconwafer and separated by an oxide tunnel barrier.

In addition to the cited oxide and ferromagnetic layers, the magnetictunnel junction (MTJ) structure can include a non-ferromagnetic metallic(ML), or seed layers, in a stacked structure. The simplest layered stackto create Perpendicular Magnetic Anisotropy (PMA) in one of the twomagnetic electrodes of a Magnetic Tunnel Junction is to form a singleferromagnetic layer over a metallic layer, and then deposit an oxidelayer over the ferromagnetic layer to give a stack designated frombottom to top ML/FML/OL, or in reverse order, OL/FML/ML.

Standard processes used in the semiconductor industry require heatingwafers up to elevated temperatures as high at 400° C. for extendedperiods of time as long as several hours in an annealing process.Therefore MTJ devices constructed through semiconductor processes mustwithstand the temperature and time used in these standard processeswithout any degradation in magnetic and/or magneto-transport properties.

The Boltzmann Factor is the probability (p) in equation (1), that athermal fluctuation causes a memory bit in an MTJ to flip between twostable states corresponding to a logical “0” and “1”. The thermalstability is related to the energy barrier between the two states (E),Boltzmann's constant (k_(B)), and the absolute temperature (T) inequation (2).

Boltzmann Factor=p(E)=e ^(−Δ)  (1)

Thermal Stability factor=Δ=E/k _(B) T   (2)

In the case of PMA, the energy barrier E depends on the magneticanisotropy of the storage (i.e. free) layer. For a uniform magnetizationreversal mechanism, the energy barrier E is proportional to the productof K_(eff)·t_(FML) where t_(FML) is the thickness of the ferromagneticlayer. K_(eff) is the effective anisotropy constant (having thedimension of an energy per unit volume).

K_(eff) can be modeled as the sum of the interfacial anisotropy andshape anisotropy.

K _(eff)=Interfacial Anisotropy+Shape Anisotropy   (3)

Interfacial anisotropy is inherent in the material properties and isrepresented by a constant K_(i) (energy per unit surface) divided by theferromagnetic layer film thickness. The shape anisotropy reduces thethermal stability and is modeled by equation (4),

Shape Anisotropy=−2πM _(s) ²   (4)

where M_(s) is the saturation magnetization, and t_(FML) is theferromagnetic layer film thickness. Interfacial Anisotropy causes PMAand the shape anisotropy reduces the PMA. In summary . . . .

K _(eff) =K _(i) /t _(FML)−2πM _(s) ²   (5)

Therefore from equation 5 the thermal stability should improve as theferromagnetic layer t_(FML) gets thinner. However, this model does notapply when t_(FML) gets below a critical thickness. Experimentationfinds that below the critical thickness, the ferromagnetic layer losesits magnetization due to imperfections and inter-diffusion withneighboring non-magnetic elements. Therefore the thermal stabilityreaches its maximum at the critical ferromagnetic thickness in a simpleML/FML/OL stack.

The simple PMA stack only provides weak PMA since there is a singleOL/FML interface. The interfacial anisotropy (K_(i)) is not strongenough to sustain PMA for ferromagnetic layers thicker than ˜15Angstroms. Moreover, there is significant inter-diffusion between theferromagnetic layer and the base metallic layer that is tantalum forexample. Inter-diffusion can cause the interface between theferromagnetic and metallic layers to be a magnetically “dead” layer. Asa result, the magnetic properties of the ferromagnetic layer are foundto degrade when t_(FML)<˜8 Angstroms. For this simple stack interfacestructure, the thermal stability at the critical ferromagnetic thicknessis only ˜0.2 erg/cm² and too small for practical applications.

An improved interface structure can be created by two OL/FML interfaces,layered in the form OL/FML/OL. This leads to higher PMA and enables theuse of a thicker ferromagnetic layer. However, it is difficult tofabricate using oxidation to form the second oxide layer without alsooxidizing the ferromagnetic layer. This leads to thick magnetically deadlayers, loss of magnetization, and an increase of the resistance—areaproduct of the Magnetic Tunnel Junction (MTJ).

Thus, an improved MTJ is needed with two oxide/FML interfaces to providehigh PMA in the reference and free layers. Furthermore, oxidation toform the upper (second) OL must be better controlled to preventundesirable oxidation of the FML and loss of PMA.

SUMMARY

The objective of the present disclosure is to provide a strongerMagnetic Tunnel Junction by strengthening the characteristics of thePerpendicular Magnetic Anisotropy in the stack structure.

A second objective of the present disclosure is to provide a method offorming the MTJ of the first objective.

According to one embodiment, the MTJ has a FML formed between two oxidelayers in a OL₁/FML/OL₂ scheme where FML has two sub-layers (FML₁, FML₂)in a FML₁/NML/FML₂ configuration where NML is a non-magnetic layer, andFML may be either a free layer or reference layer.

There are three ways the present disclosure improves the Magnetic TunnelJunction and thermal stability over the prior art. First, theresputtering of the NML having a relatively high re-sputtering rateduring the deposition of FML₂ leads to a smoother ferromagnetic layer. Asimilar concept was disclosed in related patent application Ser. No.14/939,232 with regard to depositing a second seed layer with a lowresputtering rate over a first seed layer having a high resputteringrate.

Secondly, the presence of an NML inhibits the crystallization of theFML₂. As a result, the FML₂ has smaller grains and thinner grainboundaries. This reduces the diffusion of oxygen from the top oxidelayer OL₂ to the FML₂ layer below it.

Lastly, the NML is a more highly reactive material than the FML₁ andFML₂ sub-layers. Therefore it attracts oxygen that has diffused from theOL₂ into the FML₂.

Another embodiment contains a ferromagnetic layer comprised of three FMLsub-layers and two NMLs in an alternating scheme (from bottom to top orvice versa) OL/FML₁/NML/FML₂/NML/FML₃/OL.

A third embodiment is a ferromagnetic layer comprised of a multiplicityof alternating “n+1” FML sub-layers and “n” NML layers. From bottom totop or vice versa, this stack is of the form OL/FML₁/NML₁/ . . ./FML_(n)/NML_(n)/FML_(n+1)/OL In a variation of the first, second, andthird embodiments, the OL layers at the top or bottom of the stack maybe replaced by an ML layer such as Tantalum, Tungsten, Molybdenum,Ruthenium, or Nickel-Chromium alloy. The ML/FML layer has an interfacialperpendicular magnetic anisotropy PMA.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a notional view of the prior art for a Magnetic TunnelJunction having a bottom spin valve configuration that is utilized in anMRAM, spin transfer oscillator (STO), or read/write head.

FIG. 2 is an MTJ in a prior art top spin valve configuration with thereference layer above the tunnel barrier and functionally equivalent toFIG. 1.

FIG. 3a is cross-sectional view of a free layer formed between a tunnelbarrier and an oxide capping layer in a MTJ with a bottom spin valveconfiguration wherein the free layer is a laminate comprised of anon-magnetic layer (NML) formed between two ferromagnetic layers (FMLs)according to an embodiment of the present disclosure.

FIG. 3b is a cross-sectional view of a reference layer (RL) formedbetween a seed layer and a tunnel barrier in a MTJ with a bottom spinvalve configuration wherein the RL is a laminate comprised of anon-magnetic layer (NML) formed between two ferromagnetic sub-layers(FML₁ and FML₂) according to an embodiment of the present disclosure.

FIGS. 4a and 5a represent modifications of the FIG. 3a embodimentwherein the free layer has a plurality of “n” non-magnetic layers (NMLs)in the laminated stack of NMLs and “n+1” FML sub-layers.

FIGS. 4b and 5b represent modifications of the FIG. 3b embodimentwherein the reference layer has a plurality of “n” NMLs in the laminatedstack of NMLs and “n+1” FML sub-layers.

FIG. 6 and FIG. 9 are cross-sectional views of a MTJ with a bottom spinvalve and top spin valve configuration, respectively, wherein a freelayer having a laminated stack of NMLs and FML sub-layers is formedbetween two oxide layers according to an embodiment of the presentdisclosure.

FIGS. 7-8 are cross-sectional views of a MTJ with a bottom spin valveand top spin valve configuration, respectively, wherein the free layerhas a laminated stack of NMLs and FML sub-layers formed between an oxidelayer and a non-magnetic layer according to an embodiment of the presentdisclosure.

FIG. 10 and FIG. 13 are cross-sectional views of a MTJ with a top spinvalve and bottom spin valve configuration, respectively, wherein areference layer having a laminated stack of NMLs and FML sub-layers isformed between two oxide layers according to an embodiment of thepresent disclosure.

FIGS. 11-12 are cross-sectional views of a MTJ with a top spin valve andbottom spin valve configuration, respectively, wherein the referencelayer has a laminated stack of NMLs and FML sub-layers formed between anoxide layer and a non-magnetic layer according to an embodiment of thepresent disclosure.

FIGS. 14-15 are cross-sectional views of a MTJ with a bottom spin valveand top spin valve configuration, respectively, wherein the free layeris doped with a non-magnetic material.

FIGS. 16-17 are cross-sectional views of a MTJ with a top spin valve andbottom spin valve configuration, respectively, wherein the referencelayer is doped with a non-magnetic material.

FIGS. 18-20 show a sequence of process steps during the fabrication of aMTJ with a free layer formed according to an embodiment of the presentdisclosure.

FIG. 21 shows a plot of the magnetization vs. magnetic field for variousfree layer thicknesses “t”, in Angstroms, of the prior art OL/FML/OLstack structure from FIG. 1.

FIG. 22 shows a plot of magnetization vs. magnetic field for variousfree layer thicknesses “t”, in Angstroms in aOL/FML₁(t₁)/NML/FML₂(t₂)/OL stack formed according to an embodiment ofthe present disclosure where t₁=t₂ , and t=t₁+t₂.

FIG. 23 shows a plot of magnetization vs. magnetic field for variousfree layer thicknesses “t”, in Angstroms, in a OL/FML₁ (4Angstroms)/NML₁/FML₂(t1)/NML₂/FML₃ (t₂)/OL stack formed according toanother embodiment of the present disclosure where t₁/t₂=3/4 andt=(4+t₁+t₂) Angstroms.

FIG. 24 shows a plot of the degraded magnetization vs. magnetic fieldfor a free layer stack without an NML layer that has been annealed at400° C. for five hours and illustrates that the range of FML thicknessdoes not exhibit the square loop characteristic of PMA.

FIG. 25 shows a plot of the magnetization vs, magnetic field for a freelayer stack with two NMLs that has been annealed at 400° C. for fivehours.

DETAILED DESCRIPTION

The present disclosure is a MTJ wherein at least one of a free layer,reference layer, or dipole layer has perpendicular magnetic anisotropythat is maintained during 400° C. processing of the magnetic devicessuch as embedded MRAM and STT-MRAM, in spintronic devices such asmicrowave assisted magnetic recording (MAMR) and spin torque oscillators(STO), and in various spin valve designs including those found in readhead sensors.

As disclosed in related U.S. Pat. No. 8,592,927, a MTJ may be comprisedof a pinned layer, a tunnel barrier layer, and a magnetic elementincluding a composite free layer having a magnetic saturation (M_(s))reducing (moment diluting) layer formed between two magnetic sub-layers(FM₁ and FM₂). The FM, layer has a surface that forms a first interfacewith the tunnel barrier while the FM₂ layer has a surface facing awayfrom the tunnel barrier that forms a second interface with aperpendicular Hk enhancing layer which is employed to increase theperpendicular anisotropy field within the FM₂ layer.

In related patent application Ser. No. 14/939,232, we disclosed animproved seed layer stack wherein a low resputtering rate layer withamorphous character such as CoFeB is deposited on a high resputteringrate layer that is Mg, for example, to provide a “smoothing effect” toreduce peak to peak roughness at a top surface of the uppermost NiCrseed layer in a Mg/CoFeB/NiCr configuration. Thus, the NiCr seed layerhas a smooth top surface with a peak to peak thickness variation ofabout 0.5 nm over a range of 100 nm compared with a peak to peakvariation of about 2 nm over a range of 100 nm in prior art seed layerfilms as determined by transmission electron microscope (TEM)measurements.

We have discovered that the MTJ structures disclosed in theaforementioned related applications may be further improved according tothe embodiments described herein. The MTJ in the present disclosure iscomprised of a stack structure with improved control of the oxidizationof an oxide layer above the free layer or a reference layer. The freelayer or reference layer consists of a multiplicity (n) of thinferromagnetic layers (Fe, Co, CoFe, CoFeB or combination thereof)deposited in an alternating sequence with (n−1) NMLs having a highresputtering rate and low magnetic dilution effect. According to oneembodiment, the MTJ has a FML formed between two oxide layers in aOL₁/FML/OL₂ scheme where FML has a FML₁/NML/FML₂ configuration. The roleof the NMLs is threefold and thereby provides three advantages inperformance compared with the prior art Magnetic Tunnel Junctions inFIG. 1 and FIG. 2.

First, the resputtering of the NML having a relatively high resputteringrate during the deposition of FML₂ in a FML₁/NML/FML₂ configurationleads to a smoother FML₂ ferromagnetic layer. In other embodiments,where a FML_(n) layer is deposited on a NML_(n−1) layer, a similarsmoothing effect is realized for the top surface of the FML_(n) layer.

Secondly, the presence of an NML layer inhibits the crystallization ofthe FML₂ layer, or in more general terms, a NML_(n−1) layer inhibitscrystallization in the overlying FML_(n) layer. As a result, the FML₂layer (and FML_(n) layer) has smaller grains and thinner grainboundaries. This reduces the diffusion of oxygen from the top oxidelayer OL₂ to the FML₂ layer below it.

Lastly, the NML is a more highly reactive material than the FMLsub-layers. Therefore it attracts oxygen that has diffused from the OL₂into the FML₂. As a result, the FML ferromagnetic sub-layers, andespecially the upper FML_(n) sub-layer in a stack with “n” FMLsub-layers and “n−1” NML layers, are less oxidized than in the prior artwhich leads to a better magnetoresistive ratio and greater FML thermalstability.

According to one embodiment of the present disclosure shown in FIG. 3a ,the free layer 20-1 has a FML₁/NML₁/FML₂ configuration in which FML₁ 20a made from Fe, Co, Ni, CoFe, CoB, FeB, CoFeB, CoFeNiB, or combinationthereof, is deposited on the oxide tunnel barrier layer hereafter calledthe tunnel barrier 19. The tunnel barrier is a metal oxide or oxynitridecomprised of one or more oxide or oxynitride layers made from one ormore of Si, Ba, Ca, La, Mn, V, Al, Ti, Zn, Hf, Mg, Ta, B, Cu, Cr. NML₁20 b with a thickness from 0.5 to 10 Angstroms is then deposited overthe first FML₁ 20 a. The NML₁ is a highly reactive metal with arelatively high re-sputtering rate and is typically a metal such as Mg,Al, B, Ca, Ba, Sr, Si, or C. Next a second FML₂ 20 c is deposited overthe NML₁ 20 b and is selected from one of Fe, Co, Ni, CoFe, CoB, FeB,CoFeB, CoFeNiB, or a combination thereof.

The deposition of FML₂, which has a low resputtering rate compared withNML₁, resputters a portion of NML₁, which leads to a smoother topsurface for both of NML₁ and FML₂. As described in related applicationSer. No. 14/939,232, a high resputtering rate for material A vs.material B results from one or both of a higher bond energy and a higheratomic number for material B.

The presence of NML₁ prior to the deposition of FML₂ inhibits thecrystallization of FML₂. As a result, FML₂ 20 c has smaller grains andthinner grain boundaries. This reduces the diffusion of oxygen from thesubsequently deposited capping oxide layer 40 to the FML₂ layer belowit. Furthermore, NML₁ 20 b is a more highly reactive material than theFML₂ layer. As a result, NML₁ 20 b attracts oxygen that has diffusedfrom the top oxide layer 40 into the FML₂ and thereby prevents oxidationof the FML₂.

Referring to FIG. 3b , an alternative embodiment of the presentdisclosure is depicted wherein a reference layer 10-1 having aFML₁/NML₁/FML₂ configuration is formed between a seed layer 2 and tunnelbarrier 19. The seed layer may be comprised of one or more metals oralloys such as those disclosed in related patent application Ser. No.14/939,232, or other materials used in the art.

The composition of the FML₁, NML₁, and FML₂ layers was describedpreviously. In this case, the NML₁ layer serves to prevent oxidation ofthe FML₂ layer by attracting oxygen that diffuses into FML₂ from thetunnel barrier. Otherwise, all of the benefits associated previouslydescribed with forming a FML₁/NML₁/FML₂ stack apply to the referencelayer 10-1.

According to another embodiment shown in FIG. 4a , the free layerlaminated stack 20-1 described earlier is modified to form free layer20-2 by sequentially depositing a NML₂ layer 20 d and FML₃ layer 20 e onthe FML₂ layer to give a FML₁/NML₁/FML₂/NML₂/FML₃ configuration. NML₂ isselected from one of Mg, Al, B, Ca, Ba, Sr, Si or C, and FML₃ is made ofone or more of Fe, Co, Ni, CoFe, CoFeB, CoB, FeB, and CoFeNiB. Cappinglayer 40 contacts a top surface of FML₃ 20 e. When the capping layer isan oxide, an oxide/FML₃ interface induces or enhances PMA in the FML₃layer.

In FIG. 4b , the reference layer stack 10-2 in FIG. 3b may be enhancedto form an alternative embodiment where a FML₁/NML₁/FML₂/NML₂/FML₃ stackis formed between seed layer 2 and tunnel barrier 19. In other words,additional layers NML₂ and FML₃ are sequentially deposited on FML₂ togive a reference layer having the same advantages as reference layerstack 10-1. Again, the presence of an oxide tunnel barrier 19 adjoininga top surface of the upper FML layer induces or creates PMA in the upperFML (FML₃) layer.

In FIG. 5a , another embodiment of the present disclosure is depictedwherein the free layer laminated stack 20-1 described earlier ismodified to form free layer stack 20-3 by depositing a plurality of“n−1” NML layers 20 b, 20 n−1, and “n” FML sub-layers 20 a, 20 c, 20 nin alternating fashion on the tunnel barrier 19 to give a FML₁/NML₁ . .. FML_(n−1)/NML_(n−1)/FML_(n) configuration. Each NML is selected fromone of Mg, Al, B, Ca, Ba, Sr, Si, or C, and each FML sub-layer is madeof one or more of Fe, Co, Ni, CoFe, CoFeB, CoB, FeB, and CoFeNiB.Capping layer 40 contacts a top surface of FML_(n) 20 n and may enhancePMA therein by forming an oxide layer/FML_(n) interface.

In FIG. 5b , the reference layer stack 10-1 in FIG. 3b may be enhancedto form an alternative embodiment to form reference layer stack 10-3wherein a plurality of “n−1” NML layers and “n” FML sub-layers aredeposited on seed layer 2 in alternating fashion to give a FML₁/NML₁ . .. FML_(n−1)/NML_(n−1)/FML_(n) configuration. Each NML is selected fromone of Mg, Al, B, Ca, Ba, Sr, Si or C, and each FML sub-layer is made ofone or more of Fe, Co, CoFe, CoB, FeB, CoFeB, and CoFeNiB. Tunnelbarrier 19 contacts a top surface of FML_(n) 20 n and enhances orinduces PMA therein by forming an oxide layer/FML_(n) interface. Thus,the process of depositing a FML sub-layer on a NML is repeated aplurality of times to reduce crystallization in each successive NML,provide a smoothing effect on a top surface of each FML sub-layer, andprevent oxidation of the FML_(n) by reacting with oxygen that maydiffuse from the tunnel barrier into the FML_(n).

In all of the aforementioned embodiments, the present disclosureanticipates where one or more of the FML_(n) sub-layers may be comprisedof a laminated stack such as (Co/X)_(m) or (X/Co)_(m) where m is from 1to 30, and X is Pt, Pd, Ni, NiCo, Ni/Pt, or NiFe. In another aspect,CoFe or CoFeR may replace Co in the laminated stack where R is one ofMo, Mg, Ta, W, or Cr.

Referring to FIG. 6, the present disclosure also encompasses anembodiment wherein a MTJ encompasses a free layer stack 20-1, 20-2, or20-3 formed between two oxide layers. In the exemplary embodiment, thefree layer contacts a top surface of the tunnel barrier 19, and adjoinsa bottom surface of an oxide capping layer 40 a. The oxide capping layermay be comprised of one or more oxide layers that are selected from thematerials previously described with respect to tunnel barrier 19. In abottom spin valve configuration, seed layer 2, reference layer 11, thetunnel barrier, the free layer, and capping layer 40 a are sequentiallyformed on a substrate 1 that may be a bottom electrode in a MRAM, abottom shield in a read head sensor, or a main pole layer in a STOdevice. The reference layer may be a synthetic antiparallel (SyAP)configuration wherein an antiferromagnetic coupling layer such as Ru isformed between a lower AP2 ferromagnetic layer contacting the seed layerand an upper AP1 ferromagnetic layer (not shown) contacting the tunnelbarrier. One or both of the AP2 and AP1 layers may be one or more of Co,Fe, Ni, CoB, FeB, CoFe, CoFeB, or CoFeNiB, or a laminate such as(Co/X)_(m) or (X/Co)_(m) described earlier. A top electrode 50 is formedon the capping layer and there may be an optional hard mask (not shown)such as MnPt between the capping layer and top electrode. In otherembodiments, the top electrode is a top shield in a read head sensor ora trailing shield in a STO device.

Referring to FIG. 7, an alternative bottom spin valve MTJ is shownwherein all of the layers are retained from FIG. 6 except the oxidecapping layer is replaced by a non-magnetic capping layer 40 b. In someembodiments, capping layer 40 b is one or more of Ru, W, Mo, NiCr, andTa, including Ru/Ta and Ru/Ta/Ru configurations.

In FIG. 8, a MTJ with a top spin valve configuration is shown accordingto an embodiment of the present disclosure. All layers are retained fromFIG. 7 except the positions of the free layer 20-1 (or 20-2 or 20-3) andreference layer 11 are switched so that the seed layer 2, free layer,tunnel barrier 19, reference layer, and capping layer 40 b aresequentially formed on substrate 1. The seed layer may be one or more ofW, Ru, Ta, Mo, and NiCr.

In FIG. 9, another top spin valve configuration of the presentdisclosure is depicted that represents a modification of FIG. 6 wherethe free layer 20-1 (or 20-2 or 20-3), tunnel barrier 19, referencelayer 11, and capping layer 40 b are sequentially formed on an oxidelayer 15 above an optional seed layer 2 on substrate 1. Oxide layer 15may be selected from one of the oxide materials previously mentionedwith regard to oxide capping layer 40 a. As a result, there are twooxide layer/free layer interfaces at free layer top and bottom surfaceswith tunnel barrier and oxide layer, respectively, to enhance PMA withinthe free layer.

Referring to FIG. 10, the present disclosure also anticipates thereference layer 10-1 (or 10-2 or 10-3) may be formed between two oxidelayers in a top spin valve MTJ. In the exemplary embodiment, seed layer2, free layer 21, tunnel barrier 19, the reference layer, and oxidecapping layer 40 a are sequentially formed on substrate 1. Free layer 21may be selected from the same materials as previously described withregard to reference layer 11. In this case, the reference layer has afirst interface with the oxide tunnel barrier and a second interfacewith the oxide capping layer to enhance PMA in the reference layer.

In FIG. 11, another top spin valve MTJ is shown that retains all of thelayers in FIG. 10 except the oxide cap layer is replaced with anon-magnetic capping layer 40 b described previously.

Referring to FIG. 12, a bottom spin valve MTJ is shown that retains allof the layers in FIG. 11. However, the positions of the free layer 21and reference layer 10-1 (or 10-2 or 10-3) are switched such that thereference layer, tunnel barrier 19, free layer, and capping layer 40 bare sequentially formed on seed layer 2.

In FIG. 13, another bottom spin valve embodiment is illustrated that isa modification of the MTJ in FIG. 12 where seed layer 2 is replaced byan oxide layer 15 such that the reference layer has two oxide interfacesto enhance PMA therein.

According to another embodiment shown in FIG. 14, the non-magneticmaterial that attracts oxygen from a ferromagnetic layer (FML) may beembedded or doped within the FML 22 rather than forming a laminatedstack of “n” FML sub-layers and “n−1” NMLs in earlier embodiments.Depending on the doped concentration in the FML, the non-magneticmaterial's efficiency in reacting with oxygen that may diffuse into theFML from an adjoining oxide layer may be less than in earlierembodiments involving the lamination of “n” FML sub-layers and “n−1”NMLs.

Moreover, the advantage of inhibiting crystallization in the FML mayalso be reduced compared with previous embodiments. Since a lowresputtering rate material is not deposited on a high resputtering ratematerial in this embodiment, the smoothing effect of depositing a FML ona NML described earlier does not apply here.

Free layer 22 is doped or embedded with one or more of Mg, Al, Si, Ca,Sr, Ba, C, or B where the non-magnetic material has a concentration from0.1 to 30 atomic % in the free layer. The non-magnetic material may beembedded in the free layer by a co-deposition process. The non-magneticmaterial has a magnetic dilution effect, which means that as theconcentration of the non-magnetic element is increased in the freelayer, the magnetic moment of the free layer is reduced. In theexemplary embodiment, an optional seed layer 2, reference layer 11,tunnel barrier 19, the free layer, capping layer 40 are sequentiallyformed on the substrate 1. Note that capping layer may comprise one ormore non-magnetic metals as in 40 b or an oxide material as in 40 a.

In FIG. 15, the present disclosure also encompasses a top spin valveembodiment where oxide layer 15, free layer 22, tunnel barrier 19,reference layer 11, and capping layer 40 b are sequentially formed onsubstrate 1.

FIG. 16 represents a modification of the top spin valve MTJ in FIG. 15wherein doped free layer 22 is replaced by free layer 21 describedearlier while a reference layer 12 is employed that is doped with one ormore of Mg, Al, Si, Ca, Sr, C, Ba or B. Thus, the MTJ stack has a seedlayer/free layer/tunnel barrier/doped reference layer/capping layerconfiguration.

Referring to FIG. 17, a bottom spin valve MTJ is shown where oxide layer15, doped reference layer 12, tunnel barrier 19, free layer 21, and caplayer 40 are sequentially formed on substrate 1.

The present disclosure also anticipates a method of forming a MTJwherein a ferromagnetic layer comprises a laminated stack of FMLsub-layers and NML layers as shown in FIGS. 3a -5 b. In FIG. 18, anintermediate step is shown during the fabrication of MTJ 60 that isformed by sequentially forming a seed layer 2, reference layer 11,tunnel barrier 19, free layer 20-1 (or 20-2 or 20-3), and oxide cappinglayer 40 a on substrate 1. After all of the layers in the MTJ are formedby a conventional method, a photoresist layer 55 is coated and patternedon a top surface of the cap layer 40 a to form sidewall 55s which istransferred through MTJ 60 by a subsequent ion beam etch (IBE) to formsidewall 60 s on the MTJ.

In FIG. 19, a dielectric layer 70 such as silicon oxide, silicon nitrideor alumina is deposited to a level above the capping layer, and then achemical mechanical polish (CMP) process is performed to remove thephotoresist layer and form a top surface 70 t that is coplanar with atop surface 40 t of the capping layer 40 a.

Thereafter, in FIG. 20, the top electrode 50 is formed on the dielectriclayer 70 and capping layer 40 a by a method well known to those skilledin the art.

FIGS. 21, 22, and 23 show the magnetic hysteresis loop for variousstacks that have been annealed at 330° C. for thirty minutes using Kerrmagnetometry. Magnetization is measured for fields between +1500 and−1500 Oe. Branches measured for increasing and decreasing fields areindicated as dashed and solid lines, respectively. The Kerrmagnetization signal is proportional to the perpendicular magnetization.The thickness, t, is the total thickness of one or more FML. The figuresof merit on these measurements are the squareness of the loops and thevalue of the coercive field.

The data shows the addition of one NML (FIG. 22) or two NML (FIG. 23)yields improved coercivity over a wider range in thicknesses. Inparticular, improved PMA is achieved down to layers thinner than 12Angstroms. This is contrary to the prior art without NML shown in FIG.21 for which the FML becomes discontinuous and loses its PMA below 12Angstroms.

Another benefit is improved thermal budget in a magnetic tunnel junctionhaving a free layer formed according to an embodiment described herein.FIGS. 24-25 show magnetic hysteresis loops for a stack without NML andone with two NMLs. Both stacks were annealed at 400° C. for 5 hours. Themagnetic properties of the stack without NML are strongly degraded, asindicated by the reduction of squareness and coercive field. Themagnetic signal is strongly reduced and vanishes for layers thinner than14 Angstroms. Thicker layer do not exhibit square loops characteristicof perpendicular magnetization. By contrast, the stack having 2 NMLsretains square loops and non-zero coercive fields. This indicates thatthe stack retains good PMA after 5 hour annealing at 400° C.

1. A magnetic structure comprising a ferromagnetic layer havingperpendicular magnetic anisotropy (PMA) that is formed between asubstrate and a first oxide layer (OL₁) wherein the ferromagnetic layercomprises: (a) a first ferromagnetic sub-layer (FML₁); (b) a firstnon-magnetic layer (NML₁) that is one of C, Ca, Sr, and Ba; and (c) asecond ferromagnetic sub-layer (FML₂) to give a FML₁/NML₁/FML₂configuration wherein the ferromagnetic layer (FML) contacts the OL₁ togenerate PMA in the FML layer.
 2. The magnetic structure of claim 1wherein the substrate is a second oxide layer (OL₂) that contacts abottom surface of the FML₁ layer, and the first oxide layer contacts atop surface of the FML₂ layer to give an OL₂/FML₁/NML₁/FML₂/OL₁configuration.
 3. The magnetic structure of claim 2 wherein the firstoxide layer is an Hk enhancing layer, the second oxide layer is a tunnelbarrier layer, and the ferromagnetic layer is a free layer in a bottomspin valve configuration, or a reference layer in a top spin valveconfiguration.
 4. The magnetic structure of claim 1 wherein each of theFML₁ and FML₂ sub-layers is selected from a group consisting of Fe, Co,Ni, CoFe, CoFeB, CoB, FeB, CoFeNiB, and combinations thereof.
 5. Themagnetic structure of claim 1 wherein the first oxide layer is made ofone or more of Si, Ba, Ca, La, Mn, V, Al, Ti, Zn, Hf, Mg, Ta, B, Cu, andCr.
 6. The magnetic structure of claim 2 wherein each of the first oxidelayer and the second oxide layer is made of one or more of Si, Ba, Ca,La, Mn, V, Al, Ti, Zn, Hf, Mg, Ta, B, Cu, and Cr.
 7. The magneticstructure of claim 1 wherein each of FML₁ and FML₂ has a thickness fromabout 4 Angstroms to 14 Angstroms.
 8. The magnetic structure of claim 1wherein the NML₁ layer has a thickness from about 3 Angstroms to 5Angstroms.
 9. The magnetic structure of claim 1 wherein theferromagnetic layer further comprises a second non-magnetic layer (NML₂)and a third ferromagnetic sub-layer (FML₃) to give aFML₁/NML₁/FML₂/NML₂/FML₃ configuration.
 10. The magnetic structure ofclaim 9 wherein the substrate is a second oxide layer (OL₂) thatcontacts a bottom surface of the FM₁ layer, and the first oxide layerenhances PMA in the FML₃ layer to give aOL₂/FML₁/NML₁/FML₂/NML₂/FML₃/OL₁ configuration.
 11. The magneticstructure of claim 9 wherein the ferromagnetic layer further comprisesadditional ferromagnetic sub-layers and non-magnetic layers for a totalof “s” ferromagnetic sub-layers and “s−1” non-magnetic layers formed inalternating fashion to provide a FM₁/NML₁/FM₂/NML₂/FM₃/ . . ./NML_((s−1)/)FM_(s) configuration where s≧4.
 12. A magnetic structureformed on a substrate and comprising a ferromagnetic layer havingperpendicular magnetic anisotropy (PMA) that is formed between anon-ferromagnetic metallic (NM) layer that is one of Mo, W, and NiCr,and a first oxide layer (OL₁) wherein the ferromagnetic layer comprises:(a) a first ferromagnetic sub-layer (FML₁); (b) a first non-magneticlayer (NML₁) that is one of Mg, Al, Si, C, Ca, Sr, Ba, and B; and (c) asecond ferromagnetic sub-layer (FML₂) to give a FML₁/NML₁/FML₂configuration wherein the ferromagnetic layer (FML) contacts the OL₁ togenerate PMA in the FML layer.
 13. The magnetic structure of claim 12wherein the OL1 layer contacts a bottom surface of the FML₁ layer andthe NM layer is a capping layer that contacts a top surface of the FML₂layer to give an OL₁/FML₁/NML₁/FML₂/ML configuration.
 14. (canceled) 15.The magnetic structure of claim 12 wherein the substrate is the NM layerthat contacts a bottom surface of the FML₁ sub-layer, and the firstoxide layer contacts a top surface of the FML₂ sub-layer to give aNM/FML₁/NML₁/FML₂/OL₁ configuration. 16-17. (canceled)
 18. The magneticstructure of claim 15 wherein the first oxide layer (OL₁) is made of oneor more of Si, Ba, Ca, La, Mn, V, Al, Ti, Zn, Hf, Mg, Ta, B, Cu, and Cr.19-29. (canceled)
 30. A magnetic structure formed on a substrate andcomprising a ferromagnetic (FM) layer having perpendicular magneticanisotropy (PMA) wherein the FM layer has a first surface contacting anon-ferromagnetic metallic (NM) layer and a second surface that contactsa first oxide layer (OL₁), wherein the FM layer comprises: (a) a firstferromagnetic sub-layer (FML₁); (b) a first non-magnetic layer (NML₁)that is one of Al, Si, C, Ca, Sr, and Ba; and (c) a second ferromagneticsub-layer (FML₂) to give a FML₁/NML₁/FML₂ configuration.